Although a wide range of microorganisms have been discovered that are able to degrade highly stable, toxic xenobiotics, still many pollutants persist in the environment. Recent advances in the field of r-DNA technology has provided solutions to these problems. One important factor limiting the bioremediation of sites contaminated with certain hazardous wastes is the slow rate of degradation. This slow rate limits the practicality of using bacteria in remediating contaminated sites. It is possible to extend the range of substrates that an organism can utilize. It is even possible to endow an organism with the ability to degrade a predetermined range of xenobiotics. Because biotechnological processes are based on natural activities of microorganisms and constitute variations in classic domestic waste treatment processes, they are publicly more accepted. This is an area where genetic engineering can make a marked improvement by manipulating catabolic genes of microorganisms. Advances in r-DNA technology have opened up new avenues to move toward the goal of genetically engineered microorganisms to function as "designer biocatalysts" in which certain desirable biodegradation pathways or enzymes from different organisms are brought together in a single host with the aim of performing specific detoxification. In the last 2 decades much progress has been made in this direction, and as a result catabolic genes have been cloned and characterized for organochlorines, polychlorinated biphenyls, chlorobenzoates, naphthalene etc. The aim of this review is to provide an insight in the recent advances made on characterization and expression of catabolic genes that encode the degradation/detoxification of these persistent and toxic xenobiotic compounds.